A California shark cluster of great whites spotted about 100 yards off a beach near San Francisco has prompted safety concerns for swimmers and others using the water for recreation.

U.S. Coast Guard Lt. Beau Belanger said he was "amazed" after spotting roughly 20 great white sharks near the Pacifica and Ocean Beach area on Oct. 16, said the San Francisco Chronicle. Belanger estimated the sharks in the cluster to be between 10 to 18 feet long.

"This is the first I'd heard of near-shore aggregating in such an urban area," said Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary. She said she couldn't remember a previous gathering of so many large sharks in one place.

The northeastern Pacific white sharks can top out at 21 feet long and pack on about 7,000 pounds, usually arriving from the deep ocean to feed off the Farallon Islands in the fall.

"Although salmon sharks also swim off our coast this time of year and resemble juvenile white sharks, the photos taken from 500 feet indicate they are white sharks," said David McGuire on his blog. He is director of Shark Stewards, a San Francisco shark conservation group. "It is impossible to get a frame of reference regarding size from the photographs, so we must rely on the Coast Guard’s estimate of 10-18 feet."

He said that while white sharks do not school, "feeding zones" in the area often bring them together across a 10 to 12 mile section of the San Francisco Bay coastline.

"White sharks routinely swim off the central California coastline but attacks on humans are extremely rare," said McGuire. "An unusual number of juvenile white sharks under 10 feet long have been observed this year, likely associated with the unseasonably high water temperatures along the coast."

McGuire said on the blog that those using the water should show "prudence" in avoiding shark encounters and obey posted warnings.

"There is no cause for alarm, but there is reason for caution when entering the ocean this time of year," said McGuire. "An 18-foot shark is very large, mature shark and can cause considerable harm to seals or humans."